Evolution within a niche – the accelerating mass

A selection increase in mass specific metabolism accelerated body mass evolution in horses over millions of years

The data from MacFadden’s (1986) study on fossil horses are maybe the best long-term data for unconstrained body mass evolution within a relatively stable niche. For a time span of 57 million years he lists body mass estimates for the different lineages of fossil horses, and these provide estimates of the rate of change in body mass (dw/dt in kg per million years) as a function of mass (Fig. 1, right). With an estimated dw/dt-exponent of 1.50 (SD: ± 0.17) we find that body mass evolution in fossil horses are spot on the theoretical value of 3/2 for unconstrained 2D evolution in a stable niche (Witting, 2016).

Fig. 1 The lifespan (τ, left) and body mass (w, middle) of fossil horses, as estimated for data provided by McFadden (1986). Fossil horses have a dw/dt-exponent of 1.50 (right) that coincides with a rββ/rα-ratio of infinity, as expected for within niche evolution with optimal resource handling and unconstrained selection on the pre-mass component of mass specific metabolism. From Witting (2016).

Given these data we may calculate the life history of the horse backwards for 57 million years (Fig. 1, left). Provided that a 500 kg horse today has a lifespan around 20 years, we calculate that the 25 kg horse that lived 57 million years ago had a lifespan around 90 years. If there had been no selection on process speed, we would still have 25 kg horses that live for 90 years. And if horses had increased their size due to increased resource handling alone, the 500 kg horses of today should live for 200 years.